Semiconductor memory devices, such as dynamic random access memory (DRAM) devices typically store data as a logic one or zero. The data is represented as a charge stored on a capacitor. The charge is shared with a second capacitor, such as a digit line, to produce a resultant voltage. The voltage is then compared to a reference voltage to determine the correct data state. That is, a resultant voltage greater than the reference voltage is a one, and a resultant voltage less than the reference voltage is a zero.
Prior to any memory access cycle, for normal operation of the memory and during testing of the memory, paired digit lines are equilibrated to a common potential, typically one-half the supply voltage Vcc. Memory devices include equilibration circuits for this purpose. The equilibration circuit typically comprises one or more transistors that are connected between the digit lines that form a pair of paired digit lines. These transistors are enabled by an equilibrate enable signal that is provided prior to the start of a memory access cycle. When enabled, the transistors short the paired digit lines together. In a differential sensing scheme, the digit lines are charged to opposite supply voltage potentials. Thus, the resultant equilibrate voltage is 1/2 Vcc.
When testing an integrated circuit memory device it is useful to test memory cell margins, or the amount of voltage a memory cell can move a digit line above an equilibrate voltage. As such, a memory cell with a small margin may result in erroneous data storage. This is particularly true if the memory cell is partially discharged prior to a data read operation. A separate bias voltage can be coupled to a digit line prior to reading data stored in the memory to stress cell margins. This technique is unattractive because of the relative slowness of changing the equilibrate voltage after the standard equilibrate operation is completed.
For the reasons stated above, and for other reasons which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a method and circuit for rapidly equilibrating paired digit lines of a semiconductor memory to a variable voltage, such as during testing.